A radio (wireless) communication system may be comprised of an access network and a plurality of access terminals. The access network may include access points, such as Node Bs, base stations, or the like, that allow the access terminals to connect with the access network for uplink (UL: terminal-to-network) communications and downlink (DL: network-to-terminal) communications via various types of channels. The access terminals may be user equipment (UE), mobile stations, or the like.
Although the concepts described hereafter may be applicable to different types of communication systems, the Universal Mobile Telecommunications System (UMTS) will be described merely for exemplary purposes. A typical UMTS has at least one core network (CN) connected with at least one UTRAN (UMTS Terrestrial Radio Access Network) that has Node Bs acting as access points for multiple UEs.
FIG. 1 shows the radio interface protocol architecture according to the 3GPP radio access network standards. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control information. The user plane is a region that handles traffic information with the user, such as voice or Internet protocol (IP) packets. The control plane is a region that handles control information for an interface with a network, maintenance and management of a call, and the like.
The protocol layers in FIG. 1 can be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on the three lower layers of an open system inter-connection (OSI) standard model. The first layer (L1), namely, the physical layer (PHY), provides an information transfer service to an upper layer by using various radio transmission techniques. The physical layer is connected to an upper layer called a medium access control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer. The MAC layer handles mapping between logical channels and transport channels and provides allocation of the MAC parameters for allocation and re-allocation of radio resources. The MAC layer is connected to an upper layer called the radio link control (RLC) layer, via a logical channel. Various logical channels are provided according to the type of information transmitted.
The MAC layer is connected to the physical layer by transport channels and can be divided into a MAC-b sub-layer, a MAC-d sub-layer, a MAC-c/sh sub-layer, a MAC-hs sub-layer and a MAC-m sub-layer according to the type of transport channel being managed. The MAC-b sub-layer manages a BCH (Broadcast Channel), which is a transport channel handling the broadcasting of system information. The MAC-c/sh sub-layer manages a common transport channel, such as a forward access channel (FACH) or a downlink shared channel (DSCH), which is shared by a plurality of terminals, or in the uplink the Random Access Channel (RACH). The MAC-m sub-layer may handle the MBMS data. The MAC-d sub-layer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific terminal. The MAC-d sub-layer is located in a serving RNC (SRNC) that manages a corresponding terminal and one MAC-d sub-layer also exists in each terminal.
The RLC layer, depending of the RLC mode of operation, supports reliable data transmissions and performs segmentation and concatenation on a plurality of RLC service data units (SDUs) delivered from an upper layer. When the RLC layer receives the RLC SDUs from the upper layer, the RLC layer adjusts the size of each RLC SDU in an appropriate manner based upon processing capacity, and then creates data units by adding header information thereto. These data units, called protocol data units (PDUs), are transferred to the MAC layer via a logical channel. The RLC layer includes a RLC buffer for storing the RLC SDUs and/or the RLC PDUs.
The BMC layer schedules a cell broadcast (CB) message transferred from the core network and broadcasts the CB message to terminals positioned in a specific cell or cells.
The PDCP layer is located above the RLC layer. The PDCP layer is used to transmit network protocol data, such as IPv4 or IPv6, efficiently on a radio interface with a relatively small bandwidth. For this purpose, the PDCP layer reduces unnecessary control information used in a wired network, namely, a function called header compression is performed.
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane. The RRC layer controls the transport channels and the physical channels in relation to setup, reconfiguration, and the release or cancellation of the radio bearers (RBs). The RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the UTRAN. In general, the set up of the RB refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service, and setting the respective detailed parameters and operation methods. Additionally, the RRC layer handles user mobility within the RAN, and additional services, e.g., location services.
Call setup is the process of establishing physical channels and negotiating service configuration parameters between a UE and a Node B to allow communication. Call setup procedures fall under two categories: UE originated call setup that occurs when the UE user makes a call, and UE terminated call setup that occurs when a call is made to the UE.
In UMTS, the UE may use an access channel, such as the Random Access Channel (RACH) for uplink communications. For message transmission procedures using the RACH, the following factors need to be considered.
To enable a UE to access the RACH, the UTRAN (Node B) transmits system information to all UEs within a cell. The network (UTRAN) may send such system information to the UE via a message having a Master Information Block (MIB) and System Information Blocks (SIBs). The MIB may provide scheduling information for the SIBs. Each SIB may have system information elements (IEs). The SIBs may have different characteristics related to their repetition rates, the requirements for the UEs to re-read the SIBs, and the like.
The SIBs may be broadcast throughout a cell over a broadcast channel to provide terminals (UEs) in the cell with basic system information. A terminal (ULE) may read the MIB on a broadcast control channel (BCCH) followed by the appropriate SIBs. Among the many types of SIBs, SIB type 7 (SIB7) is relevant for call setup employing RACH procedures.
After receiving the appropriate system information, the terminal sends a preamble to the network (e.g., to the Node B). This preamble contains the necessary information to allow the actual message to be sent from the terminal to the network. Namely, for call setup, the UE transmits a preamble to the access network (e.g., Node B) before sending the actual message through a random access channel (RACH).
An acquisition indication procedure for the preamble reception is performed before a message is transmitted. Here, an Acquisition Indication Channel (AICH) may be used. If an acquisition indicator signal (e.g., Acquisition Indication Channel (AICH) indicator) of the transmitted preamble does not arrive from the access network within a certain time period, the terminal increases its transmission power, re-transmits the preamble and waits to receive the acquisition indicator signal from the access network. When the terminal that has transmitted the preamble to the access network receives the acquisition indicator signal within the time period, the message is transmitted thereafter.
Additionally, the terminal can check whether or not the transmitted message is received by the access network without error upon receiving a response (ACK or NACK) for the message from the access network. Here, the response is not an acquisition indicator signal of the message, but a response signal of the message content. To transmit the response (ACK or NACK) to the message to the terminal, the upper layers of the access network performs appropriate processing.
It should be noted that the RACH is typically composed of many sub-channels defined by preamble code sequences over a timeslot. Information about the available RACH sub-channels and available preamble signatures according to ASC (Access Service Class) is included in Physical RACH (PRACH) information. According to the PRACH information, a preamble signature is assigned to a UE depending on what ASC the UE intends to use on an available RACH sub-channel.
Here, the preamble signature is chosen randomly amongst the set of available signatures. After the preamble signature is determined, access slots for the preamble are determined. When there are no available access slots, the next slot set is considered. Then, the UE transmits to the UTRAN the preamble with the determined signature in the access slots with predetermined power.
The basic concepts related to the random access channel (RACH) are known in the art of mobile communications. For example, several users can independently transmit data on a RACH, but this may result in a collision between different users. In order to reduce the probability of collisions and increase the probability of successful transmissions on a RACH, certain transmission rules are specified in order to provide waiting times to delay the transmission of data. One example of a waiting time is called a back off time, whereby the user backs off from transmitting any information on the RACH for a certain period of time.